A. Field of the Invention
The present invention relates generally to dust control filter systems, and more particularly to a control system for flushing sequentially plural filters, wherein the sequence time between flushing events is dependent upon the volume rate production of the fluid compressor and in which the sequence time may be varied by adjusting the volume rate of the fluid compressor and thus its fluid flow discharge volume.
B. Description of the Prior Art
The elimination of excess amounts of airborne dust in the workplace or in the environment has prompted increased demand upon manufacturers of surface cleaning maintenance equipment to produce equipment capable of cleaning severly contaminated surface areas. Severe dust problems are encountered in such industrial operations as cement production, foundries, dry granular chemical production, and dry granular food production. The operations of such industries generate a great deal of airborne particulate that eventually settles on the surfaces of the facility. Health and environmental considerations dictate that the surfaces must be cleaned.
Mobile rotary broom sweepers of less than about 40 horsepower have been developed for cleaning industrial facilities. Such sweepers utilize a dust and debris collecting hopper as a collecting reservoir for material swept from the contaminated surface by the broom. My co-pending U.S. patent application Ser. No. 680,942, filed Dec. 12, 1984, now Pat. No. 4,637,825, discloses a self-cleaning air filter system used in conjunction with an induced draft fan within the hopper of a mobile sweeper to control the dust during sweeping operations. The system of my co-pending application requires an air compressor providing a continuous reservoir of high pressure air, i.e. about 80 psi, to supply a filter reverse flushing cleaning system. The flushing system includes a flushing air reservoir with flushing valves in conjunction with a combined flushing control and flushing control actuator system with flushing pressure and lapse rate control such as those disclosed in my U.S. Pat. No. 4,465,497.
The application of such a sophisticated compressed air filter flushing system to rotary broom sweepers of such low horsepower and small size relative to the larger "street sweeper" class of vehicles adds to marketplace resistance relative to investment costs, operating costs, and maintenance costs. The potential of this resistance can be appreciated when one realizes the next lower level of technology for cleaning such facilities requires the investment and upkeep of a broom, shovel, and refuse barrel. The resistance to equipment cost is amply clear when one considers that it is common in the marketplace to utilize contract sweeping firms as a matter of economics relative to meeting environmental cleaning standards.
The utilization of sweepers in contract sweeping and the broad number of applications for these machines and the additional numbers of machines going into a broad range of single job severe environment applications necessitates a filter cleaning system of flexible capabilities relative to the task. Additionally, the economic resistance necessitates technology representing low initial costs, and low operating and maintenance costs.
The ability of a porous barrier filter system to gather dust is related to sustaining air flow through the filter media. Resistance to air flow through the media and the collected dust cake is generally expressed as the pressure drop across the filter media. Theoretically, the pressure drop is equal to the approach velocity to the filter media multiplied by the absolute viscosity of the conveying air stream multiplied by the mass of the dust cake divided by the permiability of the dust. Accordingly, for a given flow rate, resistance to flow increases relative to the mass of the dust cake and the time required to build the maximum desired or tolerated resistance is directly proportional to a given dust and its level of concentration. It is therefore important to select a reverse flushing cleaning frequency appropriate to sustain collecting efficiency for a given dust and its concentration.
Additionally, in a reverse flushing air filtration system, the ability of the flushing system to remove the dust cake from the face of the media is relative to the pressure potential of the air supplying the reverse flushing air blast and dwell period of the blast. The pressure requirements are relative to air reservoir volume, flushing air discharge nozzle size, and filter media area and through put air flow velocities. Presently available systems use air pressures ranging from 8 to 100 psig. The dwell period for the reverse flushing air blast is generally from 100 to 200 milliseconds with some controls allowing up to 500 milliseconds.
Typical reverse flushing systems include a common high pressure air storage reservoir supplying high volume flow to a series of quick opening filter flushing valves. Various controls have been devised to assure adequate filling of the flushing air storage reservoir and subsequent cyclic release of the compressed air through the flushing valves. Typically, the cyclic filter flushing event is controlled by a system which includes either an electrical timing control and separate means for controlling flushing air pressure or fluidic or air/mechanical systems that are energized by the flushing air source to control both time and pressure. The fluidic and air/mechanical systems generally utilize the reservoir fill time to control lapse time and pressure. The reservoir fill time is controlled by:
a. air from a volume/pressure source flowing through a fixed or variable orifice to the flushing air storage reservoir; PA1 b. air from a volume/pressure source flowing through a pressure regulator to a flushing air storage reservoir; or, PA1 c. air from a volume/pressure source flowing directly to the flushing air storage reservoir.
In addition to cost, a disadvantage of the systems of the prior art lies in their compressor requirements. All of the prior art systems have substantially more compressor capacity than is necessary for filter flushing. For example, in systems in which the air from the compressor flows through a fixed or variable orifice to the flushing air storage reservoir, the compressor at all times must operate against a back pressure at least as great as produced by the loss across the orifice. Accordingly, in such systems, compressor energy requirements and maintenance costs are high. Additionally, the prior art systems produce more compressed air than is actually necessary in order to provide sufficient flushing volume. The additional air is either stored in an auxiliary reservoir or vented. In either case, the additinal work and compressor capacity used in providing excess compressed air is or may be wasted. Examples of prior art flushing systems are disclosed in the following U.S. Pat. Nos. 2,980,207; 3,066,688; 3,332,217; 3,521,430; 3,735,566; 4,388,087; and, 4,465,497.
It is therefore an object of the present invention to provide a filter flushing control system that overcomes the shortcomings of the prior art. More specifically, it is an object of the present invention to provide a simplified filter flushing lapse time and pressure control system which provides the sophistication of variable lapse time and variable flushing pressure by varying the rate of air compressor discharge flow volume such that the time required after each flushing event to refill the flushing air reservoir to the flushing pressure is equal to the required lapse time for optimum filter efficiency. It is a further object of the present invention to provide an improved pressure operated sequencing device.